It is known that aryl groups can be chemically bound as functional groups by surface reactions to porous silicon dioxide. Conventional functional groups possessing exchange properties are then introduced onto these aryl groups. Thus, it is possible, for example, to introduce sulfonic acid groups by the sulfonation of these superficially positioned aryl groups, whereby hydrophilic silicon dioxide products are obtained having cation-exchange properties. On the other hand, anionic exchangers can be prepared in accordance with the same principle, for example by introducing chloromethyl groups into the aryl groups and coupling these with exchanging groups, e.g., amine groups.
However, according to known processes, the Si--C bond is attacked by the protons during the sulfonation reaction, and a considerable portion of the aryl groups is split off by heterolysis. Thus, the maximally attainable exchange capacity is reduced. Furthermore, ion exchange capacity additionally decreases considerably after a repeated regeneration of the exchanger with acid. These disadvantages occur also in the manufacture of corresponding anion exchangers (which has not as yet been described) so that it is impossible to obtain a high surface concentration of such ion exchanging groups.
This disadvantage can be avoided if the aryl groups are bound to the superficially located silicon atoms via CH.sub.2 -groups, rather than being bound directly.
In the process for the preparation of such cation exchangers known heretofore, described, for example, in East German Patent No. 8560, the benzylsilyl group is introduced by reacting silicon dioxide with benzyltrichlorosilane. The reaction takes place in chloroform with brief heating. The thus-obtained product, after separating the excess benzyltrichlorosilane with the exclusion of air and moisture, is sulfonated in the usual manner with chlorosulfonic acid. The thus-obtained acidic exchangers exhibit a maximum capacity of 0.5 meq./g. (milligram equivalents per gram).
It has now been found that it is possible to obtain silicon dioxide ion exchangers having considerably higher exchange capacity according to the process of this invention.